Section Did Not Pass Through the External Opening of the Cleft. the

mesoblast of the visceral folds is much more dense than that of the dorsal region of the section.

Figure 15d, as is evident, is a section through the region of the heart, which appears as three irregular cavities (ht) with fairly thick mesoblastic walls (mes′) lined with endothelium (en′). The body wall, though consisting of but little besides the ectoderm (ec), completely surrounds the heart, and the pericardial or body cavity thus formed extends dorsally as a narrow space on either side of the foregut, giving the appearance of a rudimentary mesentery, though no especial development of such a structure would naturally be expected in this region of the embryo. The foregut (ent) is a moderately large cavity lined with a very distinct entoderm of even thickness. Dorsal to the foregut are three large blood-vessels, a median, and now single, dorsal aorta (ao), and a pair of cardinal veins (cv). The notochord (nt) is small and is flattened against the ventral side of the spinal cord (sc), which latter structure needs no special mention. The muscle plates (mp) are considerably elongated, so that they now extend ventrally to a point slightly below the upper angles of the body cavity.

Figure 15e is through the middle region of the embryo, and, owing to the curvature of the body, is not an exact dorso-ventral section; this accounts, in part at least, for the unusual diameter in a dorso-ventral direction of the aorta (ao), which is very large in proportion to the other structures. The posterior cardinal vein is shown on the left, but not on the right. The relative sizes of the spinal cord (sc) and notochord (nt) are very different from what was seen in the preceding figure. In this section the spinal cord is considerably smaller than in the preceding, while the notochord is very much larger; in fact the notochord here seems abnormally large when compared to corresponding sections of other series. It is true, however, that while the spinal cord has been diminishing in diameter the notochord has been increasing. The spinal cord, notochord, and dorsal aorta are all so large that they are flattened against each other, the pushing in of the ventral side of the spinal cord being even more marked than is shown in the figure. On either side of the spinal cord a large spinal ganglion (sg) is seen, closely wedged in between the spinal cord and the adjacent muscle plate (mp). As in the preceding stage, there is a marked space between the muscle plate and the adjacent ectoderm (ec). The somatic mesoblast at the upper angle of the unenclosed body cavity is thickened on each side and somewhat bulged out by the Wolffian body to form what might be termed a Wolffian ridge (wr). In the mid-ventral line is the considerably developed mesentery (ms), from which the intestine has been torn. The Wolffian bodies now consist, on each side, of a group of five or six tubules (wt) of various sizes, near which in a more ventro-lateral position, close to the upper angle of the body cavity, is the more distinct Wolffian duct (wd). The allantois is fairly large by this time, and may be seen in the most posterior sections as an irregular, thick-walled outgrowth from the hindgut.

A horizontal section through the anterior end of an embryo of this age is shown in Figure 15f. While enclosed of course in the same membranous amnion (a), the pharyngeal region of the section is separated by a considerable space from the more anterior region where the section passes through the forebrain (fb) and eyes. The spinal cord (sc), notochord (nt), muscle plates (mp), aortæ (ao), and anterior cardinal veins (ac) need no special description. The appearance of the pharynx (ph), with its gill clefts and folds, is quite similar to that of the corresponding structures in the chick. None of the four clefts (g¹⁻⁴) show, in the plane at which the section was cut, any connection with the exterior; in fact the fourth cleft (g⁴) would scarcely be recognized as a cleft if seen in this section alone. One or two of the more anterior clefts are open to the exterior. Three pairs of aortic arches are seen, and each visceral fold has a central condensation of mesoblast.

STAGE XIII

FIGURES 16-16g (PLATES XXII., XXIII.)

The embryo (Fig. 16) now lies on one side, body torsion being complete. The curvature of the body is so marked that the exact length is difficult to determine. The eye (e) and ear (o) have about the same superficial appearance as in the preceding stage. The nose is not shown in this figure. About thirty somites are present; the exact number cannot be determined in surface view. The amnion is complete, though not shown in the figure, and the tail (t) is well formed. The umbilical stalk was torn in the removal of the embryo, so that it is not shown in the figure. The dim outline of the now convoluted heart may be seen if the “cleared” embryo be viewed by transmitted light; it is not shown in the figure. The allantois (al) is a rounded sac of considerable size just anterior to the tail. Four gill clefts (g¹⁻⁴) are now present; the most posterior one is more faint than is represented in the figure, and it could not be definitely determined from a surface view whether or not it opened to the exterior. The mandibular fold (md) is now fairly well outlined, but there is as yet no sign of the maxillary process.

Figure 16a is the most anterior of a series of transverse sections made of an embryo of the approximate age of the surface view just described; it passes through the tip of the forebrain (fb) and shows the nasal pit (n) of the right side. The great thickening of ectoderm in the region of the nasal invagination is represented by a solid line. Owing to the obliquity of the section, the left nasal pit was not cut. The mesoblast is quite dense and contains two or three small blood-vessels near the roof of the brain. The plane of this section, owing to the cranial and body flexure, cut the embryo also in the region of the pharynx; this part of the section was, as a matter of convenience, omitted from the drawing.

Figure 16b is in reality more anterior in position, considering the entire embryo, than the preceding; but the region of the embryo represented is more posterior, so that it is described at this point. The greatly elongated outline of the brain is due to its being cut through the region of flexure, so that the forebrain (fb), or, perhaps, midbrain, is shown at one end, and the hindbrain (hb) at the other. The walls of these cavities are somewhat wrinkled and irregular and their constituent cells are beginning to show slight differentiation, though this is not shown in the figure. On the left side are seen a couple of darkly stained masses; one is the origin of a cranial nerve (cn); and the other is one of the auditory vesicles (o), which is still more irregular in outline than it was in the preceding stage. The only blood-vessels to be seen are a few very small ones that lie close to the wall of the brain. The ectoderm is quite thin at all points.

Figure 16c, the largest section of this series, passes through the forebrain in the region of the eyes and through the gill clefts. The forebrain (fb) exhibits on the left a marked thickening of its wall (ch), the edge of the cerebral hemisphere of that side, which is just beginning to develop; on its right side the lower part of the forebrain is connected by a well marked optic stalk (os) with the optic cup (oc), in whose opening lies the lens vesicle (lv), now reduced to a crescentic slit by the thickening of its posterior wall. The mesoblast is more dense in those parts of the section adjacent to the pharynx than in the more distant regions, and the ectoderm thickens in a marked way as it approaches the borders of the pharynx and gill clefts. Only a few small blood-vessels (bv) are to be seen in the region of the forebrain.

Parts of three pairs of clefts (g) are shown in the figure: one pair opens widely on either side, so that there is a large area of the section that is distinct from the two still larger portions and contains a small, thick-walled cavity (g) on the right side; this cavity is a gill cleft that is cut through neither its outer nor its pharyngeal opening.

No structures other than this small portion of a gill cleft and a few blood-vessels are to be seen in this middle region of the section. In the more posterior part of the section, in which the notochord (nt) is located, a pair of curved clefts may be seen, opening entirely through the wall on the left, but closed on the right (g). One distinct pair of aortic arches is shown (ar), and also the dorsal aortæ (ao), which are of very unequal size. The spinal cord (sc) and muscle plates need no special description.

Figure 16d is in the region of the heart (ht) and lungs (lu). The former is an irregular cavity whose walls, especially on the ventral side (mes′), are becoming very thick and much folded. Although thin, the body wall completely surrounds the heart, as would be expected, since this was true of the preceding stage. The lung rudiments (lu) and the foregut from which they have arisen have the same appearance as in the chick; they consist of three small, thick-walled tubes so arranged as to form a nearly equilateral triangle. They are surrounded by a swollen, rounded mass of mesoblast which almost completely fills the surrounding portion of the body cavity (bc). The pleural sides of these crescentic portions of the body (or pleural) cavity--that is, the boundary of the mass of mesoblast just mentioned--are lined with a thickened layer of cells, shown by the solid black lines in the figure. The lung rudiments may be traced through about fifty sections of this series, or about one twelfth of the entire series. At the dorsal angle of the part of the body cavity (bc) just described, near the dorsal aorta (ao), are two dark, granular masses (ge), which, under a higher magnification than is here used, are seen to consist of a small group of blood-vessels filled with corpuscles; although several sections in front of the anterior limits of the kidneys these are evidently glomeruli. They may be traced, though diminishing in size, far toward the tail, in close connection with the Wolffian bodies. At intervals they are connected by narrow channels with the dorsal aorta; no such connection was present in the section drawn. The notochord (nt), spinal cord (sc), muscle plates (mp), and spinal ganglia (sg) need no special mention. The mesoblast is beginning to condense in the neighborhood of the notochord, and the ectoderm is slightly thickened laterally and dorsally.

Figure 16e is in the region of the liver and the Wolffian bodies; it also shows the tip of the ventricular end of the heart. The liver (li) is a large irregular mass, of a blotchy appearance under this magnification, lying between the heart (vn) and the intestine (i). Under greater magnification it is seen to be made up of indefinite strings of cells; and its still wide opening into the intestine may be seen in more posterior sections. The intestine (i), which in this section might be called the stomach, is a fairly large cavity with the usual thick entodermic walls; it is supported by a comparatively narrow mesentery. The body cavity on the side next this mesentery has the same thick lining that was noted in the region of the lungs. The convolutions of the thick peritoneal lining may easily be mistaken in places for parts of the enteron. The Wolffian bodies may be seen as two groups of tubules (wt) in their usual location. The heart is cut through the ventricle (vn), as has been said. The section being at right angles to the long axes of the villi-like growths of the myocardium, the depressions between these mesoblastic cords are seen as a number of small irregular areas, each one lined with its endocardium. The incompleteness of the body wall below the heart is apparently due to an artificial break and not to a lack of fusion. The only point that need be mentioned in connection with the structures of the dorsal part of the section is that the distinctness of the myocœl (myc) on the right side is somewhat exaggerated.

Figure 16f is in the middle region of the embryo where both splanchnopleure and somatopleure are unfused. Owing chiefly to the unclosed condition of the midgut (i) and to the increase in length of the mesentery (ms), the section is quite deep dorso-ventrally. The continuation of the amnion (a) with the somatopleure is of course here evident.

The most striking feature of the section is the marked projection of the Wolffian ridges, though no local enlargements of these ridges indicate the rudiments of the limbs. A large mass of Wolffian tubules (wt) is seen projecting into the upper part of the body cavity on each side; close to each of these masses is the posterior cardinal vein (pc), and between them is the large aorta (ao). The other structures are about as in the preceding section.

Figure 16g represents a sagittal section of the anterior half of the body of an embryo of this or possibly a slightly younger stage of development. The three regions of the brain are clearly indicated, as well as the cavity of the spinal cord (sc). The roof of the hindbrain has been made too thick in the figure; it should be represented by a mere line. A little mesoblast is to be seen at places between the roof of the brain and the superficial ectoderm. A slight invagination of the epithelium (p), between the floor of the brain and the anterior end of the notochord, probably represents the beginning of the hypophysis. No indication of the paraphysis is yet to be seen. Extending from the region of the hypophysis to the posterior end of the section is the notochord (nt); it is much vacuolated and gradually increases in thickness toward the posterior, though its outline is quite irregular; except at the extreme anterior end and at one or two other places, it lies in close contact with the floor of the neural tube. Directly under the notochord lies, in the posterior half of the figure, the large dorsal aorta (ao). The pharynx (ph), opening between the end of the forebrain and the thick mandibular fold (across which opening the amnion, a, of course extends), is a funnel-shaped space which passes out of the plane of the section toward the posterior end of the figure. Its thick endodermal lining extends to the mandibular fold on the ventral side, while on the dorsal side it gradually thins out and becomes continuous with the thin ectoderm that extends over the forebrain. Just back of the mandibular fold is the bulbus (b), and back of that is the edge of the ventricle (vn). Posterior and dorsal to the ventricle the liver (li) is seen as an irregular mass of cells, and dorsal to the liver one of the Wolffian bodies (wt) is cut through its extreme edge.

STAGE XIV

FIGURES 17-17g (PLATES XXIII., XXIV.)

Body flexure has increased until now the forebrain and tail are almost in contact (Fig. 17). The eye has developed somewhat; the ear vesicle, which is not shown in the figure, is small and seems to lie nearer the ventral side; the nasal pit is much larger and is crescentic in shape. The hyomandibular cleft (g′) still persists as a small crescentic slit, while the next three clefts are now represented merely by superficial grooves separated by distinct ridges, the visceral folds. No indication of a fifth cleft is seen. The maxillary process (mx) grows ventralward under the forebrain and is already longer than the mandibular arch (md).

The chief advance in development over the preceding stage, besides the formation of the maxillary process, is in the appearance of the appendages (aa and pa); they have the characteristic shape of the rudimentary vertebrate appendage, though the anterior pair seem to point in an unusual direction at this stage and to be slightly more developed than the posterior. The curious, anteriorly directed heart (ht) is, perhaps, somewhat abnormal. The umbilical stalk (u) is comparatively narrow and, like the allantois, was cut off close to the body.

Transverse sections of an embryo of this stage are represented in Figures 17a-g, drawn under a lower magnification than were any of the preceding figures.

Figure 17a is in the region of the pharynx, and passes through the forebrain (fb) and posterior part of the hindbrain (hb). In the thick walls of both of these structures histological differentiation has begun, so that even under low power an inner granular and an outer clear zone may be distinguished. Under greater magnification the presence of short fibers may be made out among the cells. The cerebral hemispheres (ch) are well-marked structures, their asymmetry being of course due to the obliquity of the section. Only one eye is cut by the plane of the section, and this one shows no connection with the forebrain. The outer wall of the optic cup (oc) is so thin that under this magnification it can scarcely be seen as a dark line surrounding the retinal wall. The lens (ln) is now a solid mass, of the usual type for vertebrate embryos, its front or outer wall being a scarcely discernible line. The hindbrain (hb) has the usual form for that region and does not differ particularly from what was noted in earlier stages except in the histological differentiation that has already been mentioned. As with the eye, it is only on the right side that the auditory vesicle (o) is shown. It shows some differentiation, but not so much as would be seen were it cut in another region. In the center of the section the pharynx (ph) forms an irregular cavity connected with the exterior on the left by a gill cleft (g) and by another slit which is simply the anterior margin of the stomodæum. On the right neither of these openings is in the plane of the figure, though the gill cleft (hyomandibular), which lies close to the auditory vesicle, is almost an open passage. A few small blood-vessels are scattered through the section; one of these (bv), lying between the notochord (nt) and the floor of the brain, is noticeable from its being very closely packed with corpuscles, so that at first glance, under low magnification, it looks more like a nerve than a blood-vessel.

Figure 17b is also through the pharyngeal region, a short distance behind the preceding section. The growth of the cerebral hemispheres (ch) is better shown than in the preceding figure, as is also the general form of the optic cup (oc). On the left the nasal cavity (n) is seen as an elongated slit with thick walls; it is cut near, but not through, its opening to the exterior. The same gill cleft (g) that was seen in the preceding figure is seen here as a narrow, transverse cleft, open at both ends. Between the notochord (nt) and the spinal cord (sc) is the same, though now double, blood-filled vessel (bv) that was seen in the preceding section. The other blood-vessels are larger here than in the more anterior region. There is a faint condensation of mesoblast in the neighborhood of the notochord, and a more marked condensation (mp) farther toward each side is the curiously shaped muscle plate.

Figure 17c is through the heart region, and that organ is cut through the opening from the lower or ventricular into the upper or auricular chamber. The thickening of the wall of the ventricle, which was noticed in the preceding stage, has increased to such an extent that there is now a marked difference in the thickness of the ventricular and auricular walls. As in the preceding stage, the body wall is torn, probably in handling, so that it appears to be incomplete around the ventral side of the heart. Dorsal to the heart two small circular holes (ent) with thick walls are the œsophagus and trachea, cut anterior to the point of bifurcation of the latter into the bronchial or lung rudiments. On either side of these structures is an elongated blood-vessel (ac), the anterior cardinal vein, its elongation being due to the fact that it is cut at the place where it turns downward to empty into the heart. Dorsal to the œsophagus are the aortæ (ao), which are here cut just at the point where the two vessels unite to form one; the next section, posterior to the one under discussion, shows an unpaired aorta. The notochord (nt) and spinal cord (sc) need no description, except to note that the latter shows active histological differentiation, numerous mitotic figures being seen under higher magnification, especially in the cells that line the spinal canal. On the right of the cord the edge of a spinal ganglion (sg) is seen, in connection with which in other sections are seen the clearly defined nerve roots. The condensation of mesoblast around the notochord is quite evident, and in close contact with this medial condensation are two very characteristic, S-shaped muscle plates (mp), which extend from the level of the dorsal side of the spinal cord to the upper limits of the cardinal veins. In some sections the muscle plates even yet show slight remains of the myocœl at the dorsal end.

Figure 17d is in the region of the posterior end of the heart (ht), which is cut through the tip of the ventricle, and the anterior end of the liver (li), which has the appearance of a mass of darkly stained cords or strands of cells surrounding a large blood-vessel (mv). This blood-vessel may be called the meatus venosus, though it is not separated by any line of demarcation from the auricle. A few sections anterior to this region the meatus venosus opens dorsally into a large vessel on each side (dc), which at first glance seems a part of the body cavity, but which is in reality the ductus Cuvieri, formed by the union of the anterior and posterior cardinal veins. An irregular, crescentic cleft (bc), lying medial and parallel to each of the Cuvierian vessels, is the body cavity. In the upper angle of this cavity is a granular mass, the glomerulus, that of the left side being accompanied by the extreme anterior end of the Wolffian duct. In the rounded mass of mesoblast, between the cleft-like regions of the body cavity, the lung rudiments (lu), and the œsophagus (oe) are seen as three small, circular openings; that of the œsophagus is somewhat smaller than the other two. The notochord (nt), spinal cord (sc), and muscle plates (mp) have almost the same appearance as in the preceding section. A spinal ganglion (sg) is seen on each side of the spinal cord; the one on the left shows a well-defined spinal nerve (sn), which may be traced ventrally as far as the end of the muscle plate, along whose medial side it courses. The ventral nerve root is plainly seen; the dorsal root, in this section, less plainly. The amnion (a) and abdominal wall are, as in the preceding figure, torn in the region of the ventricle.

Figure 17e is a short distance posterior to the figure just described. The liver is cut through its middle region and forms a large, darkly staining, reticular mass on the left side of the figure. The digestive tract is seen at two places to the right of the liver; the smaller and more ventral of these openings (i) may be called the intestine, while the larger is evidently the stomach (i′). The body wall is here unfused and becomes suddenly thinner as it passes upward into the amnion (a). The Wolffian tubules (wt) form a very conspicuous mass on either side of the mesentery, in close connection with the posterior cardinal veins (pc). In the mesoblast between the dorsal aorta (ao) and the notochord are two small, irregular, darkly stained masses (sy). These are shown in the preceding two figures, but were not mentioned in the description. They may be traced through a great part of the length of the embryo back of the head region; at intervals corresponding in length to the distance between the spinal ganglia they are enlarged, while between these enlargements they are very small in cross-section. At certain points a small blood-vessel is given off by the dorsal aorta to the immediate neighborhood of each of these small areas. Although they show no connection with the central nervous system, these structures appear to be the rudiments of the sympathetic nerves.

Figure 17f is in the region just in front of the hind legs. The abdominal walls are here unfused, and into the unenclosed body cavity projects the intestine (i), supported by a narrow mesentery and surrounded by a comparatively thick mass of mesoblast. The Wolffian body and duct form a mass of considerable size on each side of the mesentery. The Wolffian body is cut near its posterior end and consists of smaller tubules than in the more anterior regions. The Wolffian ducts (wd), on the other hand, are very large and are much more clearly distinguishable from the Wolffian tubules than in the preceding sections. The Wolffian ridges (wr) are very marked projections on the sides of the body, and in a region farther caudad become especially developed as the posterior appendages, to be described in connection with the following section. Both spinal ganglia are shown in this figure (sg), and in connection with the left ganglion the spinal nerve (sn), extending ventrally as far as the level of the Wolffian duct. The sympathetic nerve rudiments do not extend so far caudad as the plane of this section. The dorsal end of each muscle plate (mp) is seen, in this and other sections, to be slightly enlarged to form a round knob; this knob contains a distinct cavity (not shown in the figure), the myocœl.

In Figure 17g, owing to the curvature of the body, the plane of the section passes through the body at three places: through the region of the heart and lungs (Fig. 17d), through the region of the posterior appendages, and through the tail. In fact, the plane of the section represented by each of the preceding figures cut the embryo in more than one region, but for the sake of simplicity only one region was represented in each figure. In the figure under discussion only the leg and tail regions have been drawn, though the latter region (t), being cut through one of its curves, is seen as an elongated body with a section of the spinal cord, notochord, etc., at each end. Both regions shown in the figure are enclosed in the same fold (a) of the amnion. Of the structures in the dorsal side of the larger or more anterior part of this figure nothing need be said. The most striking feature of the section is the presence of the large posterior leg rudiments (pa). As was noted in the preceding figure, they are, as usual, merely local enlargements or projections of the mesoblast (covered, of course, with ectoblast) of the Wolffian ridge. They are, as shown in this section and in the surface view of this stage (Fig. 17), bluntly pointed projections from the sides of the body. The anterior appendage seems to be slightly more developed than the posterior, as was noted in describing the surface view of the embryo. The digestive tract is cut through its extreme posterior end, in the region that may be termed the cloaca (cl), for into it at this point the Wolffian ducts open (wdo). As the narrow cloacal chamber is followed toward the tail, it becomes still smaller in diameter, and the ventral depression or cleft seen in this figure gradually becomes deeper until its walls are continuous with the ectoderm that covers the ventral projection of mesoderm between the hind legs; no actual opening to the exterior is present, however. There is a space of about twenty-five or thirty sections (in a series of eight hundred) between the posterior ends of the Wolffian bodies and the cloacal openings of the Wolffian ducts. The body cavity (bc) and the posterior cardinal veins (pc) are very small in this region, as might be expected.

STAGE XV

FIGURE 18 (PLATE XXIV.)

Only the head of this embryo is represented, as the general state of development is about the same as in the preceding stage.

The chief object in making the figure is to show the five gill clefts (g¹⁻⁵). The fifth cleft, though small and probably not open to the exterior, is quite distinct in this embryo. The writer would feel more doubt of its being a true, though rudimentary, gill cleft had not Clarke (17) found a fifth pair of clefts in his material. The nasal pit has advanced in development somewhat and shows the beginning of the groove that connects it with the mouth. In this figure the crescentic hyomandibular cleft shows its connection with the groove between the mandibular and the hyoid folds.

STAGE XVI

FIGURE 19 (PLATE XXIV.)

This embryo is only slightly more developed than the preceding. Body flexure is so great that the forebrain and tail nearly touch. Only the anterior three gill clefts are visible. The maxillary process (mx) is longer and more narrow; the mandibular fold has not changed appreciably. The nasal pit (n) is now connected by a distinct groove with the stomodæum. The appendages have increased in size, the posterior (pa) being the longer. The anterior appendage (aa) is distinctly broadened to form the manus, while no sign of the pes is to be seen at the extremity of the posterior appendage. The heart (ht) is still very prominent. The stalk of the umbilicus (u), which is quite narrow, projects from the ventral wall in the region between the heart and the hind legs. The tail is of considerable length and is closely coiled.

STAGE XVII

FIGURES 20-20j (PLATES XXV., XXVI.)

The superficial changes noted in this stage chiefly concern the head, which has increased considerably in length (Fig. 20). The curvature of the body is slightly more marked, and the tail is more tightly coiled at the end. There are still signs of three gill clefts. The maxillary process (mx) is long and narrow, while the mandibular arch (md) is still short and broad. The fronto-nasal region has greatly increased and has the aquiline profile noted by Clarke. The nasal groove has disappeared, and there remains the small opening (n) at the side of the fronto-nasal region, near the end of the still separate maxillary process. The umbilicus is in about the same condition as in the preceding stage, but the heart is less prominent. The outline of the manus (ma) is more definite, and the extremity of the posterior appendage is distinctly flattened out to form the rudimentary pes (pe). The position of the elbow-joint in the anterior appendage is seen at the end of the reference line aa.

Typical transverse sections of this stage are shown in Figures 20a-j.

Figure 20a is a section through the middle region of the head, cutting the hindbrain on one side and the forebrain on the other. The walls of the brain show rather more histological differentiation than was seen in the preceding sections, though this cannot be shown under the low magnification used. The hindbrain (hb), which is cut near its anterior border, exhibits the usual membranous dorsal and thick ventral walls. The forebrain is here seen as three distinct cavities--a median third ventricle (tv), with a thick ventral wall, and a thin dorsal wall extended to form a large paraphysis (epi), and two lateral ventricles (ch), the cavities of the cerebral hemispheres, whose walls are quite thick except on the side next the third ventricle. The sections of this series being slightly oblique, the eye is here cut on the right side only, where it is seen as a large, semicircular cavity (e) with thick, dense walls. The mesoblast, in which several blood-vessels (bv) are seen, exhibits three distinct areas--a median, lighter zone, with a more dense area on either side. The significance of this variation in the density of the mesoblast is not apparent.

Figure 20b is only a few sections posterior to the section just described. It is drawn chiefly to show the appearance of the forebrain, the other structures being about as in the preceding figure, except that both eyes (e) are here represented. The section passes through the wide opening between the third (tv) and the lateral ventricles (ch) and cuts the anterior edge of the pineal body (epi). The paraphysis is very large and is directed backward instead of forward, as is usually the case among the lower vertebrates (if the alligator may be so classed). It is shown in Figure 17a of a preceding stage and will be again shown in a sagittal section to be described later. The same areas of more dense and less dense mesoblast noted in the preceding figure are seen here.

Subsequent investigation showed that the structure here described as the pineal body is, in reality, the paraphysis; the pineal body is absent in A. mississippiensis.

Figure 20c, though still in the head region, shows several features that were not seen in the preceding figures. On the left of the hindbrain (hb) the auditory vesicle (o), which is now considerably more advanced than in earlier figures, is seen as a larger, flask-shaped cavity and a smaller, round one. Between the larger cavity and the hindbrain is the root of a cranial nerve (cn), apparently the eighth, since in another section it comes in close contact with the wall of the larger part of the auditory vesicle just mentioned. On the right side, ventral to the hindbrain, another and much larger nerve (cn) is seen. Nearly in the center of the figure is seen a small, irregular, thick-walled cavity (p); this is the pituitary body, and its connection with the roof of the pharynx may easily be made out in another section. The mesoblast in this region of the sections contains numerous large and small blood-vessels and exhibits certain denser areas which probably represent the beginnings of the cranial cartilages. No sign of the forebrain is seen (the plane of the section passing in front of that region), except that the tip of the wall of one of the cerebral hemispheres (ch) is cut. The left nasal chamber (n) is shown: it will be noted again in the following section. The eye on the right side shows no remarkable features; its lens (ln) is large and lies well back of the lips of the optic cup, which may now be called the iris (ir). A thin layer of mesoblast has pushed in between the lens and the superficial ectoderm to form the cornea, and the outer wall of the optic cup is now distinctly pigmented. The inner wall of the optic cup is beginning to differentiate into the retinal elements. The eye on the left side is cut farther from its central region and has a very different appearance from the eye just described. This unusual appearance is due to the fact that the section passed through the choroid fissure, which is very large and seems to be formed by the pushing in of the walls of the cup and not by a mere cleft in these walls. This fissure is hardly noticeable in the stage preceding the present, and in a stage slightly older it has disappeared; so that it would seem to be a very transient structure. It apparently is formed at about the time that the optic stalk, as such, disappears. It is in the region of the choroid fissure, if not through it, that the optic nerve (on) enters the eye. Through the fissure also enters a vascular tuft of mesoblast (pt) which may be seen projecting into the optic cup after the disappearance of the fissure. This loop of blood-vessels is doubtless the pecten.

Figure 20d represents a section through the hindbrain (hb), pharynx (ph), and tip of the snout. On either side of the hindbrain are a convoluted auditory vesicle (o), and several blood-vessels and nerves, while ventral to it is seen the anterior end of the notochord (nt), around which the mesoblast is somewhat more dense than elsewhere. The pharynx (ph) sends out toward the surface a narrow gill cleft (g′) in the neighborhood of each auditory vesicle. These clefts connect with the exterior by very narrow slits, not seen in the plane of this section. The opposite end of the pharynx, as seen in this figure, opens on the left (pn) into the nasal chamber. The nasal cavity on the right is cut in such a plane that it shows neither its external nor its pharyngeal opening. The nasal passages are here fairly long and nearly straight chambers; their lining epithelium is quite thick in the middle region, but becomes thinner where it merges into the epithelium of the pharynx at one end, and into the superficial epithelium at the other end. The unusual appearance of the eye (e), on the right side of the figure, is due to the fact that the plane of the section cut tangentially through the extreme edge of the eye in the region of the choroid fissure.

Figure 20e is only a short distance posterior to the preceding. On the left side the pharynx (ph) is connected with the exterior through the stomodæum, and on the right the hyomandibular cleft (g′) is cut almost through its opening to the exterior. The auditory vesicle (o) on the right is cut near its middle region, while that on the left is barely touched by the plane of the section. The notochord (nt), with its condensed area of mesoblast, is somewhat larger than in the preceding section. The nasal canal on the right (n) is cut through neither anterior nor posterior opening, while on the left side the canal shows the anterior opening (an).

Figure 20f, which is in the region of the posterior part of the pharynx and the anterior part of the heart, shows some rather unusual conditions.

The spinal cord (sc) and notochord (nt), with the faintly outlined condensations of mesoblast in their region, need no special description. The pharynx (ph) is here reduced to an irregular, transversely elongated cavity, the lateral angles of which are connected on each side with the exterior through a tortuous and almost closed gill cleft (g), which must be followed through many sections before its inner and outer openings may be determined. Dorsal to the pharynx numerous blood-vessels (bv), both large and small, may be seen, while ventral to it is noticed a faint condensation of mesoblast (la), in the form of an inverted T, the anlage of the laryngeal structures. The ventral portion of the figure is made up of a nearly circular thin-walled cavity, the pericardium (pr). Most of the pericardial cavity is occupied in this section by the thick-walled ventricle (vn), above which is the bulbus (b) and the tip of the auricle (au). The bulbus is nearly circular in outline, though its cavity is very irregular. A few sections anterior to this, the opening of the bulbus into the ventricle is seen.

In Figure 20g the section represented is only a short distance posterior to the one represented by Figure 20f. The mesoblastic structures in the neighborhood of the spinal cord (sc) and notochord (nt) will be described in connection with the next figure, where they are more clearly defined. The œsophagus (oe)--or posterior end of the pharynx, whichever it may be called--is here a crescentic slit, with its convex side upward; ventrally it opens by a narrow glottis into the trachea (ta). The trachea is surrounded by the same condensed area of mesoblast (la) that was mentioned in connection with the preceding figure, but the condensation is here more marked. From the bulbus (b) an aortic arch (ar) extends upward for a short distance on the right side, while to the left of the œsophagus an aortic arch (ar) is cut through the upper part of its course. Ventral to the bulbus the ventricle (vn) and two auricles (au) are seen surrounded by the pericardial wall.

Figure 20h is in the region of the liver (li), which has about the same position in relation to the auricles (au) that was occupied by the ventricle in the last figure. The auricles are connected with each other by a wide passage. The trachea (ta) and the œsophagus (oe) are entirely distinct from each other; the former is a small, nearly circular hole, while the lumen of the latter is obliterated and its walls form a solid, bow-shaped mass of cells. Since there is a narrow space between this mass of cells and the surrounding mesoblast, it might be thought that the lumen of the œsophagus had been closed by the simple shrinkage of its walls; higher magnification, however, fails to show any sign of a collapsed lumen. It is doubtless the problematic and temporary closure of the œsophagus that is noticed in other forms. On each side of the œsophagus, in close relation with the anterior cardinal vein (ac), is noticed a nerve (cn) cut through a ganglionic enlargement. When traced forward these nerves are seen to arise from the region of the medulla, and when followed caudad they are found to be distributed chiefly to the tissues surrounding the newly formed bronchi; they are doubtless the tenth cranial nerves. On the right side of the figure the close connection of this nerve with the near-by gill cleft is seen. Above the paired aortæ (ao) the sympathetic nerves (sy) will be noticed. The mesoblast surrounding the spinal cord (sc) and notochord (nt) is distinctly condensed (more so than the figure shows) to form what may be called the centrum (c) and neural arch (na) of the vertebræ. The arch, owing to the slight obliquity of the section, shows here only on one side. The spinal cord is not yet completely enclosed by the neural arches. The muscle plates (mp) are in close connection with the rudiments of the vertebræ just mentioned. The spinal cord (sc) is here differentiated into three areas--a dense, deeply stained area immediately around the neurocœl; a less dense area of cells surrounding the inner area and extending ventralward as a rounded projection on each side; and an outer layer, with few or on nuclei, surrounding the inner two layers except on the dorsal side.

In Figure 20i the size and complexity of the figure are due, it will be easily understood, to the fact that the plane of the section passed through the curve of the body, thus practically cutting the embryo in two regions--an anterior, where the lungs (lu) and liver (li) are seen, and a posterior, where the Wolffian bodies (wt) are present. The spinal cord and the surrounding structures have almost the same characteristics at both ends of the figure, except that the primitive spinal column is rather more distinct in the posterior end of the section. The posterior cardinal veins (pc), Wolffian ducts (wd), and Wolffian bodies (wt) are also prominent structures of this end of the figure, the last being made up of a great number of tubules. The extreme anterior ends of the Wolffian bodies are seen in the other half of the section in the upper angles of the body cavity, dorsal to the lung rudiments (lu). Filling most of the body cavity (bc) and making up the greater part of the middle of the figure are the liver (li), now a very large organ; the stomach (i′), also quite large; the pancreas (pan), a small body lying near the stomach; and the lungs (lu), which here consist of several thick-walled tubes, surrounded by lobes of mesoblast. The other features of the figure need no special mention.

Figure 20j is through the base of the posterior appendages (pa), in which the cartilages are already being outlined by condensations of mesoblast. The intestine (i) is cut in two regions--at a more anterior point, where it is seen as a small, circular hole surrounded by mesoblast and hung by a narrow mesentery, and through the cloacal region, the larger and more ventral cavity, into which the Wolffian ducts (wd) open a short distance caudad to this section. The blood-vessels present a rather curious appearance. A short distance anterior to this point the aorta has divided into three, or it might be said that it has given off two, large branches. These two branches, one on either side near the posterior cardinal vein, pass toward the ventral side of the embryo on each side of the cloaca and end at about the region represented by the present figure. The small portion of the aorta that remains after the giving off of the two branches just described continues, as the caudal artery (ca), into the tail; it is a small vessel just under the notochord, and gives off small, paired branches at regular intervals toward the vertebral region. The posterior cardinal veins (pc), posterior to the openings of the Wolffian ducts into the cloaca, unite to form a large caudal vein lying just ventral to the caudal artery.

STAGE XVIII

FIGURE 21 (PLATE XXVII.)

This embryo, as may be seen, for example, by the form of the appendages, is slightly further developed than the one represented in Figure 20. The figure is from a photograph of a living embryo as it lay in the egg, a portion of the shell and shell membranes having been removed. The embryo, which lies on its left side, is rather faintly outlined because of the overlying allantois. The allantois has been increasing rapidly in size, and is here so large that it extends beneath the cut edges of the shell at all points except in the region in front of the head of the embryo, where its border may be seen. Its blood-vessels, especially the one that crosses the head just back of the eye, are clearly shown in the figure, and in the living specimen, when filled with the bright red blood, they form a most beautiful demonstration. As in the chick, the allantois lies close beneath the shell membranes and is easily torn in removing them.

STAGE XIX

FIGURE 22 (PLATE XXVII.)

Figure 22 is a photograph of a somewhat older embryo, removed from the egg and freed of the fetal membranes. The appendages show the position of both elbow and knee joints, and in the paddle-shaped manus and pes the digits may be faintly seen. The tail is very long and is spirally coiled, the outer spiral being in contact with the frontal region of the head. The jaws are completely formed, the upper projecting far beyond the lower. The elliptical outline of the eyes is noticeable, but the lids are still too little developed to be seen in this figure. The surface of the embryo is still smooth and white.

STAGE XX

FIGURES 23-23b (PLATE XXVII.)

In this surface view (Fig. 23) several changes are seen, though no very great advance in development has taken place. The outlines of the digits (five in the manus and four in the pes) are now well de fined; they even project slightly beyond the general outline of the paddle-shaped part. The tail has begun to straighten out, and it now extends across the front of the face. The lower jaw has increased in length, but is still shorter than the upper. The eyelids, especially the upper, are beginning to be discernible in surface view. Though still without pigment, the surface of the body is beginning to show by faint transverse lines the development of scales; these lines are most evident in this figure in the middle region of the tail, just before it crosses the nose.

A sagittal section of the entire embryo (except the tail) of this age is shown in Figure 23a. In the head region the section is nearly median, while the posterior part of the body is cut slightly to one side of the middle line. At the tip of the now well-developed snout is seen one of the nostrils (an), cut through the edge; its connection with the complicated nasal chamber (n) is not here seen, nor is the connection of the nasal chamber with the posterior nares (pn). The pharynx (ph), is anteriorly connected with the exterior through the mouth (m) and the nares, while posteriorly it opens into the œsophagus (oe); the trachea (ta), though distinct from the œsophagus, does not yet open into the pharynx. In the lower jaw two masses of cartilage are seen, one near the symphysis (mk) and one near the wall of the trachea, doubtless the rudiment of the hyoid. The deep groove back of the Meckel’s cartilage (mk) marks the tip of the developing tongue, which here forms the thick mass on the floor of the mouth cavity. Dorsal to the pharynx a mass of cartilage (se) is developing in the sphenethmoid region. This being a median section, the ventricles of the fore- (fb), mid- (mb), and hindbrain (hb) are seen as large cavities, while the cerebral hemispheres (ch) appear nearly solid, only a small portion of a lateral ventricle showing. The paraphysis (epi) is cut a little to one side of the middle and so does not show its connection with the brain. At the base of the brain the infundibulum (in) is seen as an elongated cavity whose ventral wall is in close contact with a group of small, darkly staining alveoli (p), the pituitary body. Extending posteriorly from the pituitary body is a gradually thickening mass of cartilage (bp), which surrounds the anterior end of the notochord (nt) and may be called the basilar plate. In its anterior region, where the section is nearly median, the spinal column shows its canal, with the enclosed spinal cord, while toward the posterior end of the figure the vertebræ are cut to one side of the middle line, and hence show the neural arches (na) with the alternating spinal ganglia (sg). Near the posterior end of the figure the pelvic girdle (pl) is seen. The largest organ of the embryo, as seen in this section, is the heart, of which the ventricle (vn) seems to be closely surrounded, both in front and behind, by the auricles (au). The liver (li) is the large, reticular mass back of the heart. Dorsal and anterior to the liver is the lung (lu), now of considerable size and development. The enteron is cut in several places (oe, i) and its walls are beginning to show some differentiation, though this cannot be seen under the magnification here used. One of the Wolffian bodies is seen as a huge mass of tubules (wt) extending from the pelvic region, where the mass is greatest, to the region of the lungs. The Wolffian tubules stain darkly and the whole structure forms a very striking feature of the section. Dorsal to the posterior end of the Wolffian body is a small, oval mass of very fine tubules (k), which do not stain so darkly as do the Wolffian tubules; this mass is apparently the beginning of the permanent kidney, the metanephros. Its tubules, though their origin has not been determined, seem to be entirely distinct from the tubules of the Wolffian body.

A single vertical section through the anterior part of the head of an embryo of this age has been represented in Figure 23b. On the right side the plane of the section cut through the lens of the eye (ln); on the left side the section was anterior to the lens. The upper (ul) and lower (ll) eyelids are more evident here than in the surface view. Owing to the hardness of the lens, its supporting structures were torn away in sectioning. The vitreous humor is not represented in the figure. The superior (ur) and inferior (lr) recti muscles are well shown on the right side; they are attached to the median part of a Y-shaped mass of cartilage (se), which may be termed the sphenethmoidal cartilage. Between the branches of this Y-shaped cartilage the anterior ends of the cerebral hemispheres (ch)--better called, perhaps, the olfactory lobes--are seen. Between the lower end of the sphenethmoidal cartilage and a dorsally evaginated part of the pharynx are two small openings (pn); when traced forward these tubes are found to open into the convoluted nasal chamber, while a short distance posterior to the plane of this figure they unite with each other and open almost immediately into the pharynx. The rather complicated structures of the nasal passages of the alligator have been described by the writer in another paper (57). In the lower jaw the cartilage (mk) is seen on either side and several bands of muscle are developing in the mesoblast. Two deep grooves give form to what may be called the rudimentary tongue (tn). In both jaws one or two tooth rudiments (to) may be distinguished as small invaginations of ectoderm.

STAGE XXI

FIGURE 24 (PLATE XXVII.)

In this stage the curvature of the body and tail is less marked than was seen in the last surface view. The body has increased greatly in size, so that the size of the head is relatively not so great. The size of the eye in relation to that of the head is much diminished also. The five anterior and four posterior digits are well formed, and their claws are of considerable size, though of course not present on all the digits. The outlines of scales may be traced from the tip of the tail to the skull; they are especially prominent along the dorsal profile. The skin is just beginning to show traces of pigment, which is, however, not shown in the photograph. The umbilical stalk is seen projecting with a loop of the intestine from the abdominal wall; this is shown more clearly in the next stage. The embryo now begins to exhibit some of the external characteristics of the adult alligator.

STAGE XXII

FIGURE 25 (PLATE XXVIII.)

This embryo needs no particular description. It has reached in its external appearance practically the adult condition, although there is still considerable yolk (not shown in the figure) to be absorbed, and the embryo would not have hatched for many days. Pigmentation, begun in the last stage, is now complete. The umbilical stalk is clearly seen projecting from a large opening in the body wall. The long loop of the intestine that extends down into the yolk sac is here evident, and it is hard to understand how it can all be drawn up into the body cavity when the umbilical stalk is withdrawn. No sharp shell-tooth at the tip of the snout, such as is described by Voeltzkow (78) in the crocodile, is here seen.

STAGE XXIII

FIGURE 26 (PLATE XXVIII.)

This figure shows the relative sizes of the just-hatched alligator and the egg from which it came. It also shows the position of the young alligator in the egg, half of the shell having been removed for that purpose. The blotchy appearance of the unopened egg is due chiefly to stains produced by the decayed vegetation of the nest. At hatching the young alligator is about 20 cm. long, nearly three times the length of the egg; but the tail is so compressed that, though it makes up about half of the length of the animal, it takes up very little room in the egg.

SUMMARY

Owing to the fact that the embryo may undergo considerable development before the egg is laid, and also to the unusual difficulty of removing the very young embryos, the earlier stages of development are very difficult to obtain.

The mesoderm seems to be derived chiefly by proliferation from the entoderm, in which way all of that anterior to the blastopore arises. Posterior to the blastopore the mesoderm is proliferated from the lower side of the ectoderm in the usual way. No distinction can be made between the mesoderm derived from the ectoderm and that derived from the entoderm.

The ectoderm shows during the earlier stages a very great increase in thickness along the median longitudinal axis of the embryo.

The notochord is apparently of entodermal origin, though in the posterior regions, where the germ layers are continuous with each other, it is difficult to decide with certainty.

The medullary folds have a curious origin, difficult to explain without the use of figures. They are continuous posteriorly with the primitive streak, so that it is impossible to tell where the medullary groove ends and the primitive groove begins, unless the dorsal opening of the blastopore be taken as the dividing point.

The amnion develops rapidly, and entirely from the anterior end.

The blastopore or neurenteric canal is a very distinct feature of all the earlier stages up to about the time of closure of the medullary canal.

Preceding the ordinary cranial flexure there is a sort of temporary bending of the head region, due apparently to the formation of the head-fold.

During the earlier stages of development the anterior end of the embryo is pushed under the surface of the blastoderm, and is hence not seen from above.

Body torsion is not so definite in direction as in the chick, some embryos lying on the right side, others on the left.

Of the gill clefts, three clearly open to the exterior and probably a fourth also. A probable fifth cleft was seen in sections and in one surface view.

The first trace of the urinary system is seen as a dorsally projecting, solid ridge of mesoblast in the middle region of the embryo, which ridge soon becomes hollowed out to form the Wolffian duct.

The origin of the hypophysis and paraphysis is clearly seen; the latter projects backward.

No connection can be seen between the first rudiments of the sympathetic nerves and the central nervous system.

The lumen of the œsophagus is for a time obliterated as in other forms.

The choroid fissure is a very transitory but well-marked feature of the eye.

LETTERING FOR ALL FIGURES ON PLATES VI.-XXVIII.

a, head-fold of amnion. aa, anterior appendage. ac, anterior cardinal vein. al, allantois. an, anterior nares. ao, aorta. aop, area opaca. ap, area pellucida. ar, aortic arch. au, auricle. b, bulbus arteriosus. bc, body cavity. blp, blastopore. bp, basilar plate. bv, blood-vessel. c, centrum of vertebra. ca, caudal artery. ch, cerebral hemisphere. cl, cloaca. cn, cranial nerve. cp, posterior choroid plexus. cv, cardinal veins. dc, ductus Cuvieri. e, eye. ec, ectoderm. ec′, thickening of ectoderm, en, entoderm. en′, endocardium. ent, enteron. ep, epidermal layer of ectoderm. epi, paraphysis. es, embryonic shield. f, fronto-nasal process. fb, forebrain. fg, foregut. g¹⁻⁵, gill clefts. gf¹⁻⁶, gill folds. gl, glomerulus. h, head-fold. hb, hindbrain. ht, heart. i, intestine. i′, stomach. in, infundibulum. ir, iris. it, iter. k, kidney (metanephros). l, remains of groove between secondary folds. la, larynx (cartilages of). li, liver. ll, lower lid of eye. ln, lens. lr, inferior rectus muscle of eye. lu, lungs. lv, lens vesicle. m, mouth. ma, manus. mb, midbrain. mc, medullary canal. me′, tip end of medullary canal. md, mandibular fold. mes, mesoderm. mes′, myocardium. mf, medullary fold. mg, medullary groove. mk, Meckel’s cartilage. mp, muscle plate. ms, mesentery. mv, meatus venosus. mx, maxillary fold. myc, myocœl. n, nasal invagination or cavity. na, neural arch of vertebra. nc, neurenteric canal. nl, nervous layer of ectoderm. nt, notochord. o, ear vesicle. oc, optic cup. oe, œsophagus. on, optic nerve. os, optic stalk. ov, optic vesicle. p, pituitary body. pa, posterior appendage. pan, pancreas. pc, posterior cardinal vein. pe, pes. pg, primitive groove. ph, pharynx. pl, pelvis. pn, posterior nares. pr, pericardial cavity. ps, primitive streak. pt, pecten. rt, retina. s, somites. sc, spinal cord. se, sphenethmoid cartilage. sf, secondary fold. sg, spinal ganglion. sm, splanchnic mesoblast. sn, spinal nerve. so, somatic mesoblast. st, stomodæum. sy, sympathetic nervous system. t, tail. ta, trachea. tg, thyroid gland. th, thickening and posterior limit of sf. tn, tongue. to, tooth anlage. tr, truncus arteriosus. tv, third ventricle of brain. tv′, third ventricle of brain. u, umbilical stalk. ul, upper lid of eye. ur, superior rectus muscle of eye. v′-″-‴, first, second, and third cerebral vesicles. va, vascular area. vm, vitelline membrane. vn, ventricle of heart. vv, vitelline blood-vessels. wd, Wolffian duct. wdo, opening of Wolffian duct. wr, Wolffian ridge. wt, Wolffian tubules. y, yolk.

EXPLANATION OF FIGURES 1-26 ON PLATES VI.-XXVIII.

All of the figures, with the exception of the photographs and those copied by permission from S. F. Clarke, were drawn under a camera lucida.

The magnification of each figure, except those from Clarke, is indicated below.

The photographs were made by the author, and were enlarged for reproduction by the photographic department of the Smithsonian Institution. The other surface views were made, under the author’s direction, by Miss C. M. Reese.

With the exception of Stage III., all of the figures of any one stage are given the same number, followed where necessary by a distinguishing letter, so that it is possible to tell at a glance which section and surface views belong together. The transverse sections are all cut in series from anterior to posterior.

FIGURE 1. Surface view of egg. × ²⁄₃.

1a. Egg with part of the shell removed to show the chalky band in the shell membrane. × ²⁄₃.

FIGURES 2 and 2a. Dorsal and ventral views respectively of the blastoderm before the formation of the notochord, medullary folds, etc. After Clarke.

2b-2f. Transverse sections of an embryo of the age represented in Figures 2 and 2a. × 43.

3 and 3a. Ventral and dorsal views respectively of an embryo a few days older than that represented in Figures 2 and 2a. After Clarke.

3b-3m. Transverse sections of an embryo of the age shown in Figures 3 and 3a. × 43.

FIGURES 3n and 3o. Two sagittal sections of an embryo of the same stage as Figures 3 and 3a. × 43.

4 and 4a. Dorsal and ventral views respectively of a slightly older embryo than the one shown in Figures 3 and 3a. Figure 4a shows only the head region. After Clarke.

5 and 5a. Dorsal and ventral views respectively of an embryo of almost the same age as the preceding, to show the further development of the medullary folds. After Clarke.

FIGURE 6. Dorsal view of an embryo only a day or two older than the preceding. After Clarke.

FIGURES 6a-6i. A series of transverse sections of this stage. × 43.

FIGURES 7a-7h. A series of transverse sections of an embryo slightly older than the one shown in Figures 4-6. × 43. (No surface view of this stage is figured.)

8 and 8a. Dorsal and ventral views respectively of an embryo with five pairs of mesoblastic somites. × 20. (Drawn by transmitted light.)

8b and 8c. Two sagittal sections of an embryo of this stage. × 43.

FIGURES 8d-8j. A series of transverse sections of the embryo represented in Figures 8 and 8a. × 43.

9a-9m. A series of transverse sections of an embryo somewhat more advanced in development than the one represented in the last series. × 43.

FIGURES 10 and 10a. Dorsal and ventral views respectively of an embryo with eight pairs of mesoblastic somites. × 20. (Drawn chiefly by transmitted light.)

FIGURE 11. Dorsal view of an embryo with fourteen pairs of mesoblastic somites. The area pellucida and the developing vascular area are shown, the latter having a mottled appearance. The pushing of the head under the blastoderm is also shown. × 20. (Drawn chiefly by transmitted light.)

FIGURES 11a-11k. A series of transverse sections of an embryo of this stage. × 43.

FIGURE 12. Dorsal view of an embryo with about seventeen pairs of mesoblastic somites. Part of the area pellucida is represented. (Both transmitted and reflected light were used in making the drawing.) × 13.

FIGURES 12a-12g. A series of transverse sections of an embryo of this stage. × 43.

FIGURE 13. Surface view of an embryo with about twenty pairs of mesoblastic somites. × (about) 15. (Drawn with both reflected and transmitted light.)

FIGURES 13a-13f. A series of transverse sections of an embryo slightly more developed than the one shown in Figure 13. × 20.

FIGURE 13g. A sagittal section of an embryo of about the age of the one represented in Figure 13. × 20.

14. Head of an embryo with one pair of gill clefts; ventro-lateral view. × 13.

15. Profile view of the head of an embryo with three pairs of gill clefts. × 13.

FIGURES 15a-15e. A series of transverse sections of an embryo of about the age of the one represented in Figure 15. × 20.

FIGURE 15f. A horizontal section through the anterior region of an embryo of the age of that shown in Figure 15. × 20.

16. Surface view in profile of an embryo with four pairs of gill clefts. × (about) 12.

FIGURES 16a-16f. A series of transverse sections of an embryo of the approximate age of the one represented in Figure 16. × 20.

FIGURE 16g. A sagittal section of an embryo of the age (possibly slightly younger) of the one represented in Figure 16. × 20.

17. Surface view in profile of an embryo at the time of origin of the limbs. × (about) 5.

FIGURES 17a-17g. A series of transverse sections of an embryo of the age of the one represented in Figure 17. × 7.

FIGURE 18. Surface view in profile of the head of an embryo slightly larger than, though of about the same state of development as, the one represented in Figure 17. Reproduced here chiefly to show the gill clefts. × (about) 3.

19. Surface view of an embryo somewhat more developed than the one just described. × (about) 3.

FIGURE 20. Surface view of an embryo older than the one represented in Figure 19; with well-developed manus and pes. × (about) 5.

FIGURES 20a-20j. A series of transverse sections of an embryo of the age of the one represented in Figure 20. × 7.

FIGURE 21. A photograph of a living embryo in the egg, showing the allantois, yolk mass, etc. The embryo is somewhat more developed than the one shown in Figure 20. × ²⁄₃.

22. A photograph of a still larger embryo, removed from the shell and freed from the fetal membranes. × (about) 1.

23. A photograph of a still more advanced embryo, in which the digits are quite evident and the scales are beginning to show. × (about) 1.

23a. A sagittal section of an embryo of the age of the one represented in Figure 23; the tail has not been shown in this figure. × (about) 3.

23b. A vertical section through the head of an embryo of about the size (perhaps slightly smaller) of the one shown in Figure 23. × (about) 3.

24. A photograph of an older embryo in which the pigmentation of the scales is evident, though not shown in the figure. × (about) 1.

25. A photograph of an embryo in which the pigmentation and the development of the body form are practically complete. The allantois, unabsorbed yolk, etc., have been removed. × (about) ³⁄₄.

26. A photograph of a just-hatched alligator, of an alligator egg, and of a young alligator in the egg just before hatching. × (about) ³⁄₇.

BIBLIOGRAPHY

1. ADAMS, A. L., The Wanderings of a Naturalist in India. Edinburgh, 1867.

2. ANDERSON, A., “An Account of the Eggs and Young of the Gavial (G. gangeticus),” Proc. Zoöl. Soc., 1875, p. 2.

3. BALFOUR, F. M., Comparative Embryology, vol. 2.

4. BATTERSBY, J., “Crocodile’s Egg with Solid Shell,” Nature, vol. 48, no. 1237, p. 248.

5. BISCHOFF, “Über den Bau des Crocodilherzens, besonders von C. lucius,” J. Müller’s Archiv, 1836.

6. BOAKE, BANCROFT, “The Nest of the Crocodile,” Zoölogist, vol. 5, 1870, pp. 2002-4.

7. BOETTGER, O., Katalog d. Reptilien-Samml. im Museum d. Senckenbergischen Gess., Frkft., 1893.

8. BOUTAN, LOUIS, Le Crocodile des Marias (C. palustus), Mission Scientifique Permanente d’Exploration en Indo-China Décades Zoologique, Hanoi, 1906.

9. BRANDT, , “Sur le ductus caroticus du Caiman,” Bull. Acad. St. Petersburg, vol. 17, p. 307, 1872.

10. BREHMS, Thierleben, vol. 4, pp. 498-572, 1912.

11. BRONN, H. G., Klassen und Ordnungen des Thier-Reichs, vol. 63, “Reptilien 2, Eidechsen und Wasserechsen.”

12. BRÜHL, C. B., Das Skelet der Krokodiliner, dargestellt in 20 Tafeln, 1862.

12a. BUTLER, G. W., “On the Subdivision of the Body Cavity in Lizards, Crocodiles, and Birds,” Proc. Zoöl. Soc., London, pt. 4, 1889.

13. BUTTMANN, H., De musculis Crocodili, Diss. inaugur., Hallæ, 1826.

14. Cambridge Natural History (Gadow), vol. on Amphibia and Reptiles, pp. 430-472, 1901.

15. CHAFFANJON, M. J., “Observations sur l’Alligator Mississippiensis,” Ann. de la Soc. Linn. de Lyon, vol. 28, pp. 83-96, 1881.

16. CLARKE, S. F., “The Nest and Eggs of the Alligator, A. lucius, Cuv.,” Zool. Anz., II. jahrg., no. 290, pp. 568-70, 1888.

17. ---- “The Habits and Embryology of the American Alligator,” Jour. Morph., vol. 5, pp. 182-214, 1891.

18. CLAUS and SEDGWICK, Elementary Textbook of Zoölogy. 1884.

19. COPE, E. D., Checklist of North American Batrachia and Reptiles. Washington, 1875.

19a. DAVENPORT, C. B., “Note on the Carotids and Ductus Botalli of the Alligator,” Bull. Mus. Comp. Zoöl., Harvard, vol. 24, pp. 45-51, 1893.

20. DITMARS, R. L., The Reptile Book. New York, 1907.

21. ---- Reptiles of the World. New York, 1910.

22. DUMERIL, A., and BOCOURT, ... études sur les reptiles et les batraciens du Mexique. Paris, 1870-82.

23. DUVERNOY, , “Note sur le structure du cœur des Crocodiliens,” Jour. de l’Institute, 1838.

24. EISLER, P., “Zur Kenntniss der Histologie des Alligatormagens,” Archiv. f. Mik. Anat., vol. 34, pp. 1-10, T. I., 1889.

25. FAUVEL, , An account of the Chinese alligator (title of paper not known), J. China Asiat. Soc., vol. 13, pp. 1-36, 1879.

26. FEILDEN, H. W., “The Nest of the Alligator,” Zoölogist, 2 sr., vol. 5, pp. 2090-92, 1870.

27. FELIX, W., Head-Kidney in the Crocodile. 1897.

28. GADOW, HANS, “On the Modification of the First and Second Visceral Arches, with Especial References to the Homologies of the Auditory Ossicles,” Phil. Trans., vol. 179B, pp. 451-85, T. 71-74.

29. ---- “Untersuchungen über die Bauchmuskeln der Crocodile, Eidechsen u. Schildkröten,” Morph. Jahrb., vol. 7, pp. 57-100, 1881.

29a. GEOFFROY, E., “Observations anatomiques sur le Crocodile du Nil,” Ann. Mus. Nat., vol. 2, pp. 45-47, 1803.

30. GIEBEL, C. G., “Das Skelet des westafrik. Crocodilus cataphractus,” in dessen Zeitschrift f. d. ges. Naturw., p. 105, 1877.

31. HAIR, , A thesis in the Univer. of Edinburgh (title not known), referred to by Haughton in Ann. and Mag. Nat. Hist., below.

32. HASSE, C., “Das Gehörorgan der Crocodile,” in seine Anat. Studien, p. 679, 1871.

33. ---- “Das Gehörorgan der Crocodile,” in dessen Anat. Studien, p. 679, 1873 (possibly same as no. 32).

34. HAUGHTON, S., “On the Muscular Anatomy of the Crocodile,” Proc. of the Royal Irish Acad., vol. 9, pt. 3, Dublin, 1866.

35. ---- “On the Muscular Anatomy of the Alligator,” Annals and Mag. of Nat. Hist., 4th sr., vol. 1, pp. 282-92, 1868.

36. HERRICK, C. L., “The Brain of the Alligator,” Cincinnati Acad. Nat. Hist., 1890.

36a. HERTWIG, O., Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbelthiere, vol. 2, Jena, 1906.

37. HORNADAY, W. T., The American Natural History, pp. 317-22, 1904.

38. HOWES, G. B., “On the Probable Existence of a Jacobson’s Organ among the Crocodilia,” Proc. Zoöl. Soc. London, pp. 148-59, Feb., 1891.

39. HUXLEY, THOS., “On the Dermal Armor of Jacare and Caiman, with Notes on the Specific and Generic Characters of Recent Crocodilia,” Jour. of the Proc. of the Linn. Soc., 1859.

40. JACQUART, H., “Sur plusieurs points du système veineux abdominal du Caiman à museau de brochet (Alligator lucius),” Compt. Rendus Acad. Sc. Paris, T. 47, pp. 829ff, 1858.

41. KINGSLEY, J. S., Comparative Anatomy of Vertebrates. Phila., 1912.

42. v. KLEIN, F., “Beiträge zur Osteologie der Crocodilenschädel,” Jahreshefte des Vereins f. vaterländ. Natur. in Württemberg, 19 Jahrg., p. 10, 1863.

43. KÜKENTHAL, W., “Zur Entwickelung des Handskelettes des Krokodiles,” Morph. Jahrb., vol. 19, pp. 42-55, 1892-93.

43a. LYDEKKER, RICH., New Natural History, vol. 5, 1901.

44. MAYER, , “Bemerkungen über den Schädel von Gavialis Schlegelii u. Crocodilus raninus,” Archiv. f. Naturg., p. 312, 1858.

45. MEEK, ALEX., “On the Occurrence of a Jacobson’s Organ, with Notes on the Development of the Nasal Cavity, the Lachrymal Duct, and Harderian Gland in Crocodilus porosus,” Jour. Anat. and Physiol., London, vol. 27, pp. 151-160, T. 10, 1892.

45a. MEEK, ALEX., “On the Morphogenesis of the Head of the Crocodile,” Jour. Anat. and Physiol., vol. 45, pp. 357-77, 1911.

45b. MILANI, A., “Beiträge zur Kentniss der Reptilienlunge (alligator and crocodile),” Zool. Jahrb., vol. 10, pp. 133-56, 1897.

45c. MILLER, W. S., “The Structure of the Lung,” Jour. Morph., vol. 8, p. 171, 1893.

46. OLDENBURG, HY., “Giving some Account of the Present Undertakings, Studies, and Labours, of the Ingenious in many considerable parts of the World,” Philos. Trans., vol. 1, Anno 1665 and 1666, p. 703.

47. PANIZZA, , “Sulla struttura del cuore e della circolazione del sangue del Crocodilus lucius,” Biblioteca Italiana, vol. 60, 1887 (?).

48. PARKER, W. K., On the Structure and Development of the Skull in the Crocodile, London, 1882; rev. Trans. Zoöl. Soc. London, vol. 11 or 12, pp. 263-310; Nature, vol. 26, pp. 252-54, 1882.

49. PARKER and HASWELL, Text-book of Zoölogy. 1897.

50. PETERS, W., “Über das Gehörknöchelchen und den Meckel’schen Knorpel bei den Crocodilen,” Monatsb. der königl. preuss. Akademie der Wiss. zu Berlin, p. 529, 1868.

51. ---- “Über die Höhlen des Unterkiefers bei den Crocodilen,” ibid., p. 15, 1870.

52. POELMANN, C., “Note sur le système circulatoire des Crocodiliens,” Bull. de l’Acad. Belgique, vol. 21, pp. 67-72, 1854; L’Institute, vol. 23, p. 213, 1854.

53. RABL-RÜCKHARD, “Das Centralnervensystem des Alligators,” Zeitsch. f. Wiss. Zool., vol. 30, pp. 336-73, 1878.

54. RATHKE, C., Untersuchungen über die Entwickelung und den Körperbau der Crocodile. Braunschweig, 1866.

55. REESE, A. M., “The Nasal Passage of the Florida Alligator,” Proc. Phila. Acad. Nat. Sci., pp. 457-64, 1901.

56. ---- “Artificial Incubation of Alligator Eggs,” Amer. Nat., vol. 35, no. 411, pp. 193-95, March, 1901.

57. ---- “A Double Embryo of the Florida Alligator,” Anat. Anz., vol. 28, nos. 9 and 10, pp. 229-31, 1906.

58. ---- “The Development of the American Alligator,” Smithsonian Misc. Coll., no. 1791, pp. 1-66, 1908.

59. ---- “The Breeding Habits of the Florida Alligator,” ibid., no. 1696, pp. 381-86, 1907.

60. ---- “The Home of the Alligator,” Pop. Sc. Monthly, pp. 365-72, Oct., 1910.

61. ---- “Development of the Digestive Canal of the American Alligator,” Smith. Misc. Coll., no. 1946, pp. 1-25, 1910.

62. ---- “The Development of the Brain of the American Alligator: The Paraphysis and Hypophysis,” Smith. Misc. Coll., no. 1922, pp. 1-20, 1910.

63. ---- “The Histology of the Enteron of the Florida Alligator,” Anat. Record, vol. 7, no. 4, pp. 105-29, April, 1913.

64. ---- “The Vascular System of the Florida Alligator,” Proc. Phila. Acad. Nat. Sc., pp. 413-25, 1914.

65. RÖSE, C., “Über die Zahnentwickelung der Krokodile,” Verhandl. d. anat. Gesellsch., Jena, vol. 6, pp. 225-27, 1892; also Morph. Arb., Jena, vol. 3, pp. 195-228, 1893-4.

66. ---- “Über die Verwachsung von retinierten Zähnen mit den Kieferknochen,” Anat. Anz., pp. 82-89, 8 Jahr, 1893.

67. ---- “Über das rudimentäre Jacobsonschen Organ der Krokodile und des Menschen,” Anat. Anz., pp. 458-72, 8 Jahr, 1893.

68. ---- “Über die Nasendrüse und die Gaumendrüsen von Crocodilus poroso,” Anat. Anz., vol. 8, pp. 745-51, 1893.

69. ---- “Über die Zahnleiste und die Eischweile der Sauropsiden,” Anat. Anz., vol. 7, pp. 748-64, 1892.

70. SCHWALBE, G., “Über Auricularhöcker bei Reptilien, ein Beitrag zur Phylogenie des Äusseren Ohres,” Anat. Anz., vol. 6, pp. 43-53, 1891.

71. SCLATER, W. L., List of the Reptiles and Batrachians of S. Africa. 1898.

72. SLUITER, E. P., “Das Jacobsonsche Organ von Crocodilus porosus (Schn.),” Anat. Anz., vol. 7, pp. 540-45, 1892.

73. SMITH, H. M., “Notes on the Alligator Industry,” Bull. U. S. Fish Com., vol. 11, pp. 343-45, 1891.

74. STEVENSON, C. W., “Utilization of the Skins of Aquatic Animals; Leather from Alligator Skins,” Report of Com. Fish and Fisheries for 1902, pp. 342-46, 1904.

75. TENNENT, Sir J. E., Sketches of the Natural History of Ceylon. London, 1861.

76. VAN BEMMELEN, J. F., “Die Visceraltaschen und Aortenbogen bei Reptilien und Vögeln,” Zool. Anz., vol. 9, pp. 528-32, 543-46, 1886.

76a. VIRCHOW, HANS, “Ueber die Alligatorwirbelsäule,” Archiv. fur Anat., parts 2 and 3, pp. 103-142, 1914.

77. VOELTZKOW, A., “On the Oviposition and Embryological Development of the Crocodile,” translated in Ann. Nat. Hist., vol. 9, pp. 66-72, 1891.

78. ---- “Biologie und Entwickelung der äusseren Körperform von Crocodilus madagascariensis Grand,” Abhandl. Senckenberg Naturf. Gesell., vol. 26, part 1, pp. 1-149, 1889.

79. VOELTZKOW, M. (?), “Beiträge zur Entwickelungsgeschichte der Reptilien, I-IV (Crocodilus madagascariensis und Podocnemis madagascariensis),” ibid., vol. 26, 1889.

80. VROLIK, , “Sur le cœur du Caiman à museau de Brochet (Crocodilus lucius),” Het Instituut, 1841.

81. WAYTAILINGAM, S., “Notes on the Breeding of Crocodilus Palustris,” Proc. Zoöl. Soc. London, pp. 186-87, 1880.

82. WIEDERSHEIM, R., Comparative Anatomy of Vertebrates. 1899.

83. ---- “Beiträge zur Entwickelungsgeschichte des Urogenitalapparatus der Krokodile und Schildkröten,” Verhndl. der 10. internat. Med. Cong. Berlin, vol. 2, pt. 1, pp. 132-34, 1891; Anat. Anz., 5 Jahrg., pp. 337-44, 1890.

84. WILLISTON, S. W., Water Reptiles of the Past and Present. Chicago, 1914.

85. ZIEGLER, H. E., Lehrbuch der vergleichenden Entwickelungsgeschichte der niederen Wirbelthiere. Jena, 1902.

86. ZUCKERHANDL, E., “Zur Anatomie und Entwickelungsgeschichte der Arterien des Unterschenkels und des Fusses,” Anat. Hefte, pt. 1, vol. 5, pp. 207-91, 1895 (?).

87. ---- “Zur Anatomie des Vorderarmes” (2d part), ibid., pp. 157-205, 1895.

88. ---- “Description anatomique de trois crocodiles envoyés de Siam par les Pères Jésuites,” Mémoires de l’académie Royale des sciences, vol. 3, part 2, p. 266.

89. ---- Description anatomique d’un crocodile, ibid., part 3, p. 173.

INDEX

Abdominal aorta and branches, 212

Abdominal ribs, 80

Acetabulum, 85

Air chamber of egg, 228

Albumen of egg, 229, 230

Allantois, 299, 300, 328

Alligator, 6 abundance of, 8 American, 3 and cane rat, 28 and muskrat, 28 attack from, 14 bellowing of, 18 catching of, 34 cave of, 12 Chinese, 38 Cynocephalus, 110 daylight hunting of, 33 dealers in, 34 derivation of name of, 40 differs from crocodile, 7 digging from cave, 33 distribution of, 10 economic importance of, 26 eggs used as food, 35 feeding of, 12 fire hunting of, 32 habitat of, 8 hatching of, for sale, 35 hibernation of, 12, 13 hides, annual output of, 28 for card cases, etc., 30 chief centers for, 27 damaged in removal, 31 Floridian, 28, 29 highest priced, 32 length and width of, 30 from Louisiana, 28, 29, 30 methods of cutting, 31 Mexican, 29, 30 removal of, 31 salting of, 31 shipment of, 31 from South and Central America, 27, 28 from Southern States, 28 varieties of, 29 value of different sizes of, 28, 29 value to hunter of, 28 hole, 11 hunting, 32 Joe, 9 killing of, 33 for sport, 27 laws for protection of, 28 leather, first used, 26 for shoes, 27 imitation, 32 present use of, 32 meat, preparation of, 35 smoking of, 36 use as food, 35 mississippiensis, 3, 7 raw hides, selling of, 32 sale of live, 34 sinensis, 3, 16 the stuffing of, 34 swimming of, 13, 14 tanned hides, sale of, 32 teeth, sale of, 34 value of, 34 trail, 11, 13 unknown to ancients, 40 use of tail, 14 value of live, 34

Alligatoridæ, 1

Amnion, 236, 247, 251, 259, 266, 267, 268, 269, 270, 274, 275, 278, 290, 334

Ampullæ, 149

Ancestry, 4

Annulus tympanicus, 149

Aortic arches, 203, 296, 299

Appendages, development of, 308, 315, 317, 327, 328

Appendicular skeleton, 81

Area opaca, 233

Area pellucida, 233

Arkansas Alligator Farm, 201

Arterial system, 212 first reference to, 44

Arteries, of anterior region, 215, 216 brachial, 218 caudal, 216 cervical, 220 cloacal, 316 cœliac, 212 collateralis colli, 217, 219 common carotid, 219, 221 crural, 216 dorsal aorta, 212 fibular, 214 first hæmorrhoidal, 216 gastric, 212 gastro-hepatico-intestinal, 212 iliac, 214 inferior dental, 223 injection of, 201 internal carotid, 70, 221 internal mammary, 217 ischiadicæ, 214 lingual, 220 lumbar, 213 mandibular, 220 mesenteric, 213 œsophageal, 217 pancreo-intestinal, 212 pelvic, 214, 216 pleural, 217 of posterior region, 213 primary carotid, 203, 218, 220 pulmonary, 203 radial, 218 rectal, 216 right subclavian, 203, 216 sciatic, 214 second hæmorrhoidal, 216 spleno-intestinal, 212 subclavian, left, 219 subscapular, 217 superior dental, 223 thoracic, 218 thyroid, 217 tibial, 214 ulnar, 218 urogenital, 214 vertebral, 217

Arytenoid cartilage, 147

Atlantosaurus, 4

Atlas, 53

Auditory capsule, 72

Auditory vesicle, 274, 277, 286, 294, 297, 302, 309, 320, 322

Auricles, 204

Axis, 54

Bartram’s account, 8

Basilar plate, 331

Belly skin, 31

Belodon, 5

Bile duct, 154

Bird and crocodile, 40, 41

Blastopore, 233, 234, 235, 236, 240, 246, 249, 250, 252, 257, 263, 264, 272

Body cavity, development, 279, 281, 287

Body flexure, 307, 317, 318

Bones, alisphenoid, 68 angular, 76 of anterior limb, 82 articular, 75 basilingual plate, 76 basioccipital, 67, 68, 72 basisphenoid, 67 calcaneum, 88 centrale, 84 clavicle, 82 coracoid, 81, 82 coronoid, 75 dentary, 74 epiotic, 73 epipubis, 86 episternum, 82 exoccipital, 70 fibula, 87 fibulare, see calcaneum of foot, 88 frontal, 60 humerus, 82 hyoid, 76 ilium, 84 integumental, 50 interclavicle, 81 ischium, 85 jugal, 62, 67 lachrymal, 62 malar, 62 maxilla, 61, 64 mesethmoid, 72 metacarpals, 84 nasal, 60 opisthotic, 73 palatine, 65 parietal, 59 of pelvic girdle, 84 pisiform, 83 of posterior limb, 84 postfrontal, 59 prefrontal, 60 premaxilla, 61, 63 pro-otic, 73 pterygoid, 66, 70 pubis, 86 quadrate, 62, 69 quadratojugal, 64, 67 radius, 83 scapula, 81 splenial, 75 squamosal, 59 supra-angular, 75 supraorbital, 62 suprascapula, 81 tarsalia, 88 tibia, 87 tibiale-centrale, 88 transpalatine, 66 ulna, 83 ulnare, 80 vomer, 72

Brain, 132

Breeding habits of alligator, 18

Bronchial rings, 199

Buttons, 27

Caiman, 36 of Amazon, 37 banded, 3 black, 3, 36 latirostris, 3 niger, 3, 7 palpebrosus, 3 round-nosed, 3 sclerops, 3 species of, 37 spectacled, 3, 37 teeth of, 36 trigonotus, 3 ventral armor of, 36

Capitulum of rib, 78

Carpus, 83

Cauda equina, 131

Cement, 65

Centrum, development of, 325

Cerebellum, 132, 133

Cerebral hemispheres, 132, 133, 134 development of, 302, 309, 310, 332

Cerebral peduncles, 133, 134

Cerebral vesicles, 266, 273, 319

Cervical cord, 132

Chalky band of egg, 229

Chewing muscles, 90

Chinese alligator, 3

Chorda, tympani, 137

Choroid, 147

Choroid fissure, 321, 322, 337

Clarke, S. F., 226, 227, 228, 230, 231, 232, 233, 236, 243, 247, 250, 274, 293, 317, 318

Classification, 1

Claws, 46, 84 development of, 333

Cleavage of mesoblast, 263

Clitoris, 196

Cloaca, 155 embryonic, 316, 327

Cloacal glands, 156

Cocoa, Fla., 27

Columella, 74, 149

Conjunctiva, 136

Conjunctival gland, 146

Conus arteriosus, 203

Copulation of crocodile, 195

Copulatory organs, 194

Cornea, 146 development of, 321

Corn marks, 29, 30

Cornua of hyoid, 76

Corpora cavernosa, 194

Cranial cartilages, 320

Cranial flexure, 273, 276, 283, 291

Cranial nerves, 132, 135, 302, 320, 325

Cranium, 58

Cricoid cartilage, 197

Crocodile--Crocodilus, 6 African, 39 caves of, 41 distribution of, 40 egg laying of, 41 held sacred, 40 in Madagascar, 40 mentioned by Herodotus, 40 Voeltzkow’s account, 40 American, 2, 37 colors of, 39 distribution of, 38 Ditmars’ experience, 38 range of, 37 cataphractus, 2 Cuban, 2, 39 derivation of name of, 40 Guatemala, 2 intermedius, 2, 39 johnstoni, 2 Madagascar, 2 eggs of, 228 hatching of, 41 nest of, 41 man-eating, 40, 43 marsh, or mugger, 42 moreletti, 2 Nile, 21, 39 niloticus, 2, 39 Orinoco, 2, 39 palustris, 3, 42 migration of, 42 porosus, 2 rhombiferus, 2, 39 robustus, 2 rough-backed, 3 salt-water, 2, 42 in captivity, 43 habitat, 43 size of, 42 skeleton of, 51 sharp-nosed, 2 Siamese, 2 swamp, 3

Crocodilia, 1

Crocodilidæ, 1, 2, 3

Deaths by crocodiles in Africa, 40 in India, 43

Deltoid ridge, 83

Dentine, 65

Dermal skeleton, 47

Diaphragm, 115

Digestive system, 152

Digestive tract, fixation of, 159 histology of, 189 outline of, 158

Digits, development of, 329, 332

Dinosauria, 4

Dorsal aorta, development of, 277, 278, 327

Dorsal fissure, 131, 132, 133

Dorsal shield, 47

Drum, 148, 149

Ductus Cuvieri, 312

Ear, 147, 148

Ectoderm, 233

Eggs, 227, 231 artificial incubation of, 24 incubation of, 22 number of, per nest, 21, 22, 23 shape of, 228 shell of, 228 size of, 25, 227, 228 taken from oviduct, 24 variation in size of, 26 weight of, 25

Elizabeth Thompson Science Fund, 226

El lagarto, 40

Embryo, development of, 231 earliest stages of, 232, 233 position of, 230 removal of, from egg, 231 stages: I., 233; II., 235; III., 240; IV., 247; V., 249; VI., 257; VII., 266; VIII., 267; IX., 273; X., 282; XI., 293; XII., 293; XIII., 300; XIV., 307; XV., 316; XVI., 317; XVII., 318; XVIII., 328; XIX., 328; XX., 329; XXI, 333; XXII., 334; XXIII., 334

Embryology, summary of, 335

Enamel, 65

Endoskeleton, 50

Enteron, development of, 261, 262, 269, 271, 278, 287

Entoderm, 234

Epidermal skeleton, 46

Epiglottis, 197

Episternum, 81

Epitrichial cells, 48

Eustachian tube, 72, 149

Everglades, 10, 220

External auditory meatus, 70, 73, 148

External mandibular foramen, 75

Extracolumellar cartilage, 74

Eye, 144 glands of, 144

Eyeball, 146

Eyelids, 144 development of, 329, 332

Feeding of alligators, 15

Fenestra ovalis, 73

Fissura ventralis, 131

Food of alligators, 15

Foramen ovale of skull, 68

Forebrain, 274, 276, 284, 291, 294, 302, 308, 319

Foregut, 240, 248, 252, 297

Foreskin, 195

Fort Pierce, Fla., 27

Fourth ventricle, 132

Fronto-nasal region, 318

Fundic region of stomach, 152

Gastroliths, 44, 45, 153

Gavial, 6 food of, 44 Indian, distribution of, 43 eggs and nest of, 44 meaning of, 44

Gavialidæ, 1, 3

Gavialis gangeticus, 2 character of, 43, 44 size of, 43

Genital ducts, 156

Geographical distribution of Crocodilia, 6

Gescmackwärzchen, 165

Gharial, 44

Gill clefts, 277, 283, 285, 293, 294, 299, 301, 302, 303, 316, 317, 318, 323, 336

Gizzard, 153

Glans penis, 195

Glenoid cavity, 81

Glomeruli, 304

Glottis, development of, 324

Growth of alligators, 16

Hallux, 88

Harderian gland, 136, 145

Head-fold, 233, 236, 237

Heart, 202, 204 development of, 267, 270, 279, 283, 287, 297, 303, 310

Hindbrain, 277, 284, 294, 308, 319

Histology of enteron, 157 of integument, 48

Horn alligator, 31

Horny layer, 48

Hyoid, 151 development of, 330

Hyomandibular cleft, 286, 307, 309, 322

Hypophysis, 53, 133 development of, 306, 320, 331, 337

Incubation, period of, 25

Infundibulum, 133, 134 development of, 330

Integument, histology of, 48

Internal auditory meatus, 73

Internal mandibular foramen, 76

Intestine, 154 development of, 289

Iris, 146 development of, 321

Jacksonville, Fla., 34

Kidneys, 192

Kissimmee, Fla., 27

Labyrinth, 148

Lachrymal canal, 145

Lachrymal gland, 144, 145

Lake Kissimmee, 10

Lake Worth, 37

Large intestine, epithelium of, 187 histology of, 186 see Rectum

Larynx, 197 development of, 323

Lateral disks of stomach, 153

Lateral ventricle, developing, 319

Laying season of alligator, 18

Lens, 147

Lens vesicle, 276, 284, 294, 302, 309, 321, 332

“Leviathan” of Book of Job, 40

Liver, 154 development of, 304, 307, 312, 326

Lower jaw, 74

Lungs, 199, 200 capillaries of, 200 development of, 303, 304, 312, 331

Mandible, 74

Mandibular fold, 296, 301, 308, 317, 318

Manus, 84, 317, 318, 329

Mating season of alligator, 19

Maxillary process, 301, 308, 317, 318

Meatus venosus, 312

Meckel’s cartilage, 330

Medulla, 132

Medullary canal, 254, 255, 258, 259, 260, 262, 267, 268, 269, 270, 271, 272, 275, 282, 291

Medullary folds, 250, 251, 253, 254, 256, 258, 265, 266, 291 origin of, 336

Medullary groove, 236, 237, 238, 241, 242, 244, 245, 248, 250, 253, 255, 256, 263, 265, 313, 334, 336, 337

Medullary plate, 249

Melbourne, Fla., 27

Mesentery, development of, 299

Mesoderm, 234

Metanephros, 332

Miami, Fla., 27

Midbrain, 275, 276

Middle ear, 149

Mouth, 150

Muscles: abdominal, 112 ambiens, 118 anconæus, 104 atlanti-mastoideus, 96 capiti-sternalis, 96 caput coraco-scapulare, 105 caput humerale mediale, 106 caput humerale posticum, 105 caput humeri laterale, 105 caput scapulare laterale, 104 carpo-metacarpalis, 111 carpo-metacarpalis V., 111 carpo-phalangei, 109, 110 carpo-phalangeus, 111 carpo-phalangeus primus digiti V., 111 caudali-ilio-femoralis, 120 caudi-femoralis, 121 cerato-hyoideus, 93 cervicalis adscendens, 96 collo-capitis, 94 collo-occipitis, 96 collo-scapularis superficialis, 97 collo-squamosus, 95 collo-thoraci-suprascapularis profundus, 98 coraco-antebrachialis, 101 coraco-brachialis, 101 coraco-ceratoideus, 92 costo-coracoideus, 93, 99 costo-scapularis, 94 costo-vertebralis lateralis, 94 costo-vertebralis medialis, 94 deltoideus scapularis inferior, 103 diaphragmatic, 115 dorsalis scapulæ, 103 of dorsal neck region, 94 dorso-humeralis, 102 dorso-scapularis, 97 episterno-ceratoideus, 92 epistropheo-vertebralis, 95 extensor hallucis proprius, 129 extensor ilio-tibialis, 118 extensor longus digitorum, 125 of eyeball, 146 femoro-tibialis, 119 flexor digitorum brevis, 128 flexor longus digitorum, 127 flexor tibialis externus, 122 flexor tibialis internus, 122 of forearm, 107 gastrocnemius, 126 humero-antebrachialis inferior, 102 humero-carpi-radialis, 108 humero-carpi-ulnaris, 108 humero-metacarpalis, 108 humero-radialis, 106 humero-radialis brevis, 108 humero-radialis internus, 107 humero-radialis lateralis, 109 humero-radialis longus, 107 humero-radialis medialis, 109 humero-ulno-phalangei, 110 ilio-femoralis, 120 ilio-fibularis, 119 ilio-ischio-caudalis, 130 intercostales, 115 intermaxillaris, 91 interosseus cruris, 128 ischio-femoralis, 123 latus colli, 92 maxillo-coracoideus, 93 maxillo-hyoideus, 93 metacarpo-phalangeus, 111 metacarpo-phalangeus I., digiti V., 112 obliquus abdominis externus, 112 obliquus abdominis internus, 113 occipito-cervicalis medialis, 94 occipito-epistropheus, 96 occipito-maxillaris, 91 pectoralis, 100 pectoralis minor, 100 peroneus anterior, 125 pisiformi phalangeus primus digiti V., 111 of posterior appendages, 118 pterygo-maxillaris, 91 pubi-ischio-femoralis externus, 123 pubi-ischio-femoralis internus, 124. pubi-ischio-femoralis posterior, 124 quadratus lumborum, 115 rectus abdominis, 113 rectus internus, 115 rectus lateralis, 114 rectus ventralis, 113 retractor oculi, 137 rhomboideus, 99 of scapula, 96 scapulo-humeralis profundus, 103 sphincter colli, 91 squamoso-cervicalis medialis, 95 sterno-atlanticus, 97 subscapularis, 104 supracoracoideus, 100 of tail, 129 temporalo-maxillaris, 90 teres major, 103 tibialis anticus, 125 tibialis posticus, 128 transversus abdominis, 113 ulno-carpi-radialis, 109 ulno-radialis, 107 of ventral side of neck, 91

Muscle plates, 289, 297, 311, 325

Musk glands, 156

Myocardium, development of, 305

Myocœl, 256, 280, 281, 282, 311

Nasal passages, 151

Nasal pit, 294, 307, 310, 317, 322, 330

Nephrostome, 290

Nerves abducens, 133, 134, 135, 137 acoustic, 133, 137 alveolar branch, inferior, 136 axillaris, 141 brachialis longus inferior, 141 brachialis longus superior (radialis), 141 brachial plexus, distribution of, 140 coraco-brachialis, 141 crural and ischiadic plexuses, 142, 143 cutaneus brachii et antebrachialis medialis, 141 cutaneus pectoralis, 141 dorsalis scapulæ (posterior), 141 facial, 133, 137 frontal branch, 136 glossopharyngeal, 134, 137 hypoglossal, 68, 134, 138 latissimi dorsi, 141 nasal branch, 136 oculomotor, 68, 133, 134, 135 olfactory, 135 optic, 68, 134, 135 pectoralis, 141 pneumogastric, see vagus postsacral, 142, 143 presacral, 142, 143 sacral, 143 scapulo-humeralis profundus, 141 spinal (1-4), 138, 139, 140 subscapularis, 141 supracoracoideus, 140 teres major, 141 thoraci inferiores, 140 trigeminal, 68, 133, 135, 136, 137 trigeminal, inferior maxillary branch, 136 trigeminal, ophthalmic branch, 136 trigeminal, superior maxillary branch, 136 trochlear, 133 vagus, 68, 134, 137

Nervous epithelium of ear, 149

Nervous layer of ectoderm, 259, 268

Nervous system, 131

Nest of alligator, compactness of, 21 construction of, 21 form of, 21 location of, 20 size of, 21 temperature in, 24

Neural arches, development of, 325, 331

Neural groove, see Medullary groove

Neurenteric canal, 264, 267, 272, 275, 282, 336

New York Zoölogical Park, crocodilians in, 39

Nictitating membrane, 144

Notochord, 236, 238, 245, 248, 249, 251, 255, 256, 260, 263, 266, 269, 270, 285, 295, 306, 335

Nuchal shield, 47

Obex, 133

Oblique muscles, 146

Odontoid process, 52, 54

Œsophagus, 151, 152, 324, 330 cilia of, 174 epithelium of (feeding), 173 epithelium of (hibernating), 172 histology of, 168 transsections of (figures), 169, 170

Okefinokee, 10, 226

Olfactory bulb, 132, 133

Olfactory lobes, development of, 332

Olfactory tract, 132, 133, 134

Olivary enlargement of œsophagus, 151

Optic chiasma, 134 cup, 302, 309, 310, 321 lobes, 132, 133, 135 nerve, development of, 321 stalk, 302 tracts, 134 vesicle, 274, 276, 282, 294

Oral cavity, 150

Ora serrata, 147

Osteolæmus tetrapis, 3

Otic vesicle, see Auditory vesicle

Outer ear, 148

Ova, 193

Ovary, 193

Oviducts, 156, 193, 194

Palm Beach, Fla., 9

Pancreas, 154 development of, 326

Papillæ of tongue, 150

Paraphysis, 132, 133 development of, 319, 320, 330

Pecten, 147, 321

Pectoral girdle, 81

Penis, shaft of, 194

Pericardium, 323

Periotic capsule, 73

Pes, 317, 318, 329

Petromyzon marinus, 157

Pharynx, development of, 277, 285, 292, 295, 299

Pigment, 333, 334

Pineal body, 132, 319

Pits in scales, 49

Pituitary body, see Hypophysis

Plover and crocodile, 40

Posterior cardinal vein, development of, 298

Postorbital bar, 60

Prickle cells, 49

Primitive groove, 246, 249, 250, 256, 265, 267

Primitive spinal column, 326

Primitive streak, 233, 240, 246, 249, 250, 256, 265, 267, 275

Procœlia, 1

Pulp cavity of tooth, 65

Pupil, 146

Recessus cavi tympani, 149

Recessus scalæ tympani, 149

Rectum, 155 transsection of (fig.), 186

Rectus muscles of eye, 146 development of, 332

Respiratory organs, 197, 200; (fig.), 198

Rete Malpighii, 48

Retina, 147 development of, 321

Retractor oculi muscle, 146

Ribs, 77

Rima auditoria, 148

Ring muscle, 157

Roof of mouth, 165 covering of (fig.), 166 glands of, 167 papillæ of, 167

Saccus naso-lachrymalis, 146

Scales, development of, 333

Sclera, 146

Scutes, 47

Semicircular canals, 149

Semilunar valves of stomach, 154

Sexual characteristics, 19

Sexual maturity, 17

Shell membrane, 228

Shell-tooth, 334

Sinus venosus, 202

Size of alligator, 16 at hatching, 16

Skeleton, 46, 50

Skin, 87

Skull, 58 dorsal aspect of, 59 lateral aspect of, 67 posterior aspect of, 70 sagittal section of, 72 ventral aspect of, 63

Smaller part of stomach, 153

Small intestine, 154 histology of, 179, 186 mucosa of (fig.), 184 transsection of (figs.), 181, 182, 183, 185

Smithsonian Institution, 226, 227

Somatapleure, 279

Somites, 251, 252, 256, 266, 267, 274, 282, 292, 300

Special sense organs, 144

Spinal cord, 131, 290, 291, 298, 311, 325 development of, 298

Spinal ganglion, development of, 299, 311, 312, 314, 331

Spinal nerves, 138

Splanchnopleure, 279

Stenosauria, 1

Sternum, 77, 80

Stomach, 152, 153 development of, 305, 326 glands of, 177, 179 histology of, 174 transsection of (fig.), 176

Stomodæum, 317, 322

Stratum corneum, 48

Supratemporal fossa, 57

Sympathetic nerves, 314, 325, 337

Systemic arch, 203

Tail, 317, 318, 329

Tailfold, 274

Tapetum lucidum, 146

Tarsus, 88

Taste papillæ, 165

Tear dots, 145

Teeth, 47, 64

Teleosauria, 1

Temperature range in swamps, 23

Tendon Achilles, 126, 127

Testes, 194

Thoracic ribs, 78

Thyroid gland, development of, 78, 269

Tomistoma schlegeli, 2 skeleton of, 51

Tongue, 150 covering of (fig.), 160, 161 development of, 333 epithelium of, 161 glands of, 160, 162, 163, 164 histology of, 157 papillæ of, 160, 165

Tooth, development of, 333 socket of, 64 structure of, 65

Torsion of body, 274, 336

Trachea, 197 development of, 324, 330 rings of, 198, 199

Trigeminal foramen, 78

Tuberculum of rib, 78

Tympanic cavity, 70, 73, 148, 149

Umbilical stalk, 300, 308, 317, 318, 333, 334

Ureter, 156, 193 development of, 290

Urogenital organs, 192, 196

Valves of outer ear, 148

Vasa deferentia, 156

Vascular system, 201 lettering for, 224

Veins, of anterior region (fig.), 209 anterior vena cava, 208 axillary, 210 brachial, 210 caudal, 207 coronary, 202 external jugular, 211 femoral, 206 hepatic, 202, 205 hepatic portal, 205 iliac, 206 inferior dental, 211 internal epigastric, 206 internal jugular, 208 internal mammary, 208 ischiadic, 207 lingual, 211 mesenteric, 205 muscular, 211 pancreatic, 205 of pes, 207 postbrachial, 210 postcaval, 202, 204 of posterior region, 204 precaval, 202, 208 pulmonary, 203 radial, 211 rectal, 207 renal portal, 207 subclavian, 210 subscapular, 210 superior dental, 211 thoracic, 210 vertebral, 208

Velum palatinum, 151

Venous system, 204

Ventricle, 203

Vertebræ, cervical, 51, 52 caudal, 57 lumbar, 56 sacral, 56

Vertebral column, 50

Vertebrarterial canal, 78

Vitelline blood-vessels, 267, 283

Vitelline veins, 270

Vitreous humor, 332

Vocal cords, 197

Voeltzkow, A., 228, 230, 231, 232, 334

Voice of alligator, 18 before hatching, 25

Warts, 49

Wolffian body, 289, 299, 304, 305, 307, 314, 316, 326, 331, 336 ducts, 280, 281, 289, 299, 314, 316 ridge, 299, 306, 314 tubules, 306, 313, 331

Xiphisternal horns, 81

Yolk of egg, 230

Transcriber’s Notes

Language inconsistencies and errors in spelling, capitalisation and hyphenation and in the translations of terms have been retained, except as mentioned under Changes below. The textual differences between the list of illustrations and the illustration captions have not been standardised.

Page 123, paragraph Pubi-ischio-femoralis Externus: (Plate III., Fig. 1, p. is. f. int., Plate IV., Fig. 1, p. is. f. int.) should possibly read (Plate III., Fig. 1, p. is. f. ext., Plate IV., Fig. 1, p. is. f. ext.).

Page 273-274: ... to the region of the midbrain (v²) ...: v² does not occur in Plates XVIII. or XIX.; ... at the side of the forebrain (v′) ...: v′ occurs in Plate XVII.

Some of the numbers in round brackets refer to the bibliography at the end of the text. In some bibliographical references the source document shows blank spaces rather than first names.

Changes made

Footnotes and illustrations have been moved out of text paragraphs; some ditto marks have been replaced with the dittoed text.

The references to plates (for example those after page 130) have been standardised regarding the use of commas and italicised abbreviations.

Some minor typographical and punctuation errors have been corrected silently, as have some minor lay-out and typographical inconsistencies.

Page vii, items 10 and 11: captions of individual photographs have been added.

Page 3: repeated footnote marker removed from last column of Caiman niger.

Page 51: ... skeleton of Tomistoma schlegali ... changed to ... skeleton of Tomistoma schlegeli ....

Page 72, caption with Fig. 24: supraangular changed to supra-angular.

Page 87, caption with Fig. 29: tb¹ changed to tb′ cf. illustration.

Page 107: entsprect changed to entsprecht.

Page 165: Gescmackwärzchen changed to Geschmackwärzchen.

Page 258: ... as shown in Figures 9 and 9b. changed to ... as shown in Figures 9a and 9b.

Page 343, item 12: Das Skelet der Krokodiliner changed to Das Skelet der Krokodilinen.

Page 345, item 36a: experimentallen changed to experimentellen; item 47: struttura del cruore changed to struttura del cuore.

Page 347, item 67: des Menchen changed to des Menschen; item 70: Auricular höcker changed to Auricularhöcker; item 80: Het. Institute changed to Het Instituut.

Index: some spellings corrected to reflect the spelling as used in the text (coranoid to coronoid, Cyanocephalus to Cynocephalus, Gescmackwärzchen to Geschmackwärzchen).